Today we are going to share a very classic interview question about horses. Whether it is school recruitment, or social recruitment, in various companies have appeared, I have also asked others.

Let’s cut to the chase.

01. Examples of topics

25 horses
There is a field with 25 horses, and the field has 5 tracks. No timer is used for the race (i.e. only the order of the race is available for each race).

What is the minimum number of races needed to pick the three fastest horses? And give the analysis process!

02. Topic analysis

I really don’t want to be one of those people who have to grind through their analysis and then throw out the right answer. The answer is 7 times, know how to leave, qualified today, we do not waste time. Those of you who are confused and those of you who are wrong look down, and those of you who only know three horses look down.


Analysis process:

  • 5 times: First we divided 25 horses into 5 groups (A, B, C, D, E) and ran 5 times to get the first place in each group.


  • 1 time: then we let these 5 first place run once, get the top three of them. Note: here you get A1, the fastest horse of all. In addition, the group where D1 and E1 reside can be eliminated. Second and third places are definitely not in it!


  • 1 times: since we have finished first, A1 does not need to participate in the race. Meanwhile, D1 and E1 are eliminated from the group. C1 takes first place in group 3, and no one in group C runs faster than C1. And B2 is probably third faster than C1. Similarly, A2 and A3 could be faster than B1 and B2. So for the seventh race, we have A2, A3, B1, B2 and C1. (Beg everybody don’t blame me wordy,, I am really worried that there are so few students don’t understand…)


In the end, we finished in the top three out of 25 horses in seven races.

03. Upgrade versions

25 horses. What’s the minimum number of races we have to run to find the top five fastest horses? (Here I want to say, I see a lot of places on the Internet to get this question wrong, so if you can’t, please take a look carefully.)


In the above analysis, we have identified the no. 1. However, the second and third places can be generated in A2-A3-B1-B2-C1, so we need to discuss them separately.


  • Suppose the second and third are respectively A2 and A3

    In this case, the fourth place might be A4, in which case the fifth place would be A5 or B1. The fourth place could be B1, and the fifth place could be B2 or C1. So we only need [A4, B1, A5, B2, C1] to enter the competition once and get the top five.

  • If the two and three are respectively: A2, B1

    In this case, the fourth place might be A3, B2, C1. Assume that number 4 is A3, and number 5 might be A4, B2, C1. Assume that number 4 is named B2, and number 5 May be A3, B3, or C1. Assume that number 4 is C1, and number 5 might be A3, B2, C2, D1. At this point we need at least two races to find the fourth and fifth place in [A3, A4, B2, B3, C1, C2, D1], so we need nine.


Other possibilities include:

  • Suppose the two and three are respectively: B1, A2
  • Suppose the two and three are respectively: B1, B2
  • Suppose the two and three are respectively: B1, C1


The analysis methods of the above three cases are the same, so I don’t want to explain it clearly. The general idea is that we need to analyze the possible fourth place according to the third place. Then according to the fourth, analyze the corresponding situation of the fifth. Finally, among these horses, the true fourth and fifth place are chosen.


Because they’re asking you the minimum number of races you can finish in the top five. So according to the analysis, if the second and third places were A2 and A3, it would only take eight runs to get out of the top five. The minimum number is 8. (This question is not rigorous, so if an interviewer asks it, it’s best to give a derivation of all possibilities.)


I’ve seen a lot of respondents give an 8 at the beginning of this question, but they don’t say why. Before I went to the electricity school recruitment, also asked a student this question, the other side came up to give me an 8, asked its process, a face mengbi. I hope those of you who read this article, the next time you are asked this question, will simply give all the analysis and hang up the interviewer. After all, we who know are so plain and boring.


So, did you get today’s quiz question? Leave your thoughts in the comments section!

I’ve compiled all the problems I’ve written into an e-book, complete with illustrations for each problem. Click to download

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